Predicting the maximum aa/Ap index through its relationship with the preceding minimum

Abstract

Abstract. Predicting the strength and peak time of geomagnetic activity for the ensuing cycle 25 is important in space weather service for planning future space missions. The minimum aa geomagnetic index around the solar minimum has been often used to predict the maximum amplitude of sunspot cycle, but seldom used to directly predict the maximum aa index. This study analyzed the relationships between the maxima and minima of both the geomagnetic aa and Ap indices for the 11-year cycle. The maximum aa index is found to be well correlated to the preceding minimum with a correlation coefficient of r = 0.860. As a result, the maximum aa index for the ensuing cycle 25 is predicted to be aamax(25) = 26.9 ± 2.6. This value is equivalent to Apmax(25) = 17.3 ± 1.8 ± 1.2 if employing the high correlation between aa and Ap (r = 0.939). The maximum Ap index is also found to be well correlated to the preceding minimum with a correlation coefficient of r = 0.862. Based on this correlation, the maximum Ap index is predicted to be a slightly higher value of Apmax(25) = 19.0 ± 1.6. The rise time of the aa (Ap) index for the 11-year cycle is found to be nearly uncorrelated to the following maximum, r = −0.16 (−0.17). If the data point for cycle 24 (which is far from others) were not considered, the rise time of the Ap index for the 11-year cycle would be weakly correlated to the following maximum, r = −0.404 at a confidence level of 62 %. The rise time for cycle 25 would be roughly estimated to be 89.9 ± 31.6 (months), implying that the geomagnetic activity for the ensuing cycle 25 would peak around April 2025 ± 32 months.